40,859 research outputs found
On the Hierarchy of Block Deterministic Languages
A regular language is -lookahead deterministic (resp. -block
deterministic) if it is specified by a -lookahead deterministic (resp.
-block deterministic) regular expression. These two subclasses of regular
languages have been respectively introduced by Han and Wood (-lookahead
determinism) and by Giammarresi et al. (-block determinism) as a possible
extension of one-unambiguous languages defined and characterized by
Br\"uggemann-Klein and Wood. In this paper, we study the hierarchy and the
inclusion links of these families. We first show that each -block
deterministic language is the alphabetic image of some one-unambiguous
language. Moreover, we show that the conversion from a minimal DFA of a
-block deterministic regular language to a -block deterministic automaton
not only requires state elimination, and that the proof given by Han and Wood
of a proper hierarchy in -block deterministic languages based on this result
is erroneous. Despite these results, we show by giving a parameterized family
that there is a proper hierarchy in -block deterministic regular languages.
We also prove that there is a proper hierarchy in -lookahead deterministic
regular languages by studying particular properties of unary regular
expressions. Finally, using our valid results, we confirm that the family of
-block deterministic regular languages is strictly included into the one of
-lookahead deterministic regular languages by showing that any -block
deterministic unary language is one-unambiguous
Unambiguous Languages Exhaust the Index Hierarchy
This work is a study of the expressive power of unambiguity in the case of automata over infinite trees. An automaton is called unambiguous if it has at most one accepting run on every input, the language of such an automaton is called an unambiguous language. It is known that not every regular language of infinite trees is unambiguous. Except that, very little is known about which regular tree languages are unambiguous.
This paper answers the question whether unambiguous languages are of bounded complexity among all regular tree languages. The notion of complexity is the canonical one, called the (parity or Rabin/Mostowski) index hierarchy. The answer is negative, as exhibited by a family of examples of unambiguous languages the cannot be recognised by any alternating parity tree automata of bounded range of priorities.
Hardness of the examples is based on the theory of signatures, previously studied by Walukiewicz. The technical core of the article is a definition of the canonical signatures together with a parity game that compares signatures of a given pair of parity games (of the same index)
Relationships Between Bounded Languages, Counter Machines, Finite-Index Grammars, Ambiguity, and Commutative Equivalence
It is shown that for every language family that is a trio containing only semilinear languages, all bounded languages in it can be accepted by one-way deterministic reversal-bounded multicounter machines (DCM). This implies that for every semilinear trio (where these properties are effective), it is possible to decide containment, equivalence, and disjointness concerning its bounded languages. A condition is also provided for when the bounded languages in a semilinear trio coincide exactly with those accepted by DCM machines, and it is used to show that many grammar systems of finite index — such as finite-index matrix grammars (Mfin) and finite-index ET0L (ET0Lfin) — have identical bounded languages as DCM. Then connections between ambiguity, counting regularity, and commutative regularity are made, as many machines and grammars that are unambiguous can only generate/accept counting regular or com- mutatively regular languages. Thus, such a system that can generate/accept a non-counting regular or non-commutatively regular language implies the existence of inherently ambiguous languages over that system. In addition, it is shown that every language generated by an unambiguous Mfin has a rational char- acteristic series in commutative variables, and is counting regular. This result plus the connections are used to demonstrate that the grammar systems Mfin and ET0Lfin can generate inherently ambiguous languages (over their grammars), as do several machine models. It is also shown that all bounded languages generated by these two grammar systems (those in any semilinear trio) can be generated unambiguously within the systems. Finally, conditions on Mfin and ET0Lfin languages implying commutative regularity are obtained. In particular, it is shown that every finite-index ED0L language is commutatively regular
Ambiguity Hierarchy of Regular Infinite Tree Languages
An automaton is unambiguous if for every input it has at most one accepting
computation. An automaton is k-ambiguous (for k>0) if for every input it has at
most k accepting computations. An automaton is boundedly ambiguous if there is
k, such that for every input it has at most k accepting computations. An
automaton is finitely (respectively, countably) ambiguous if for every input it
has at most finitely (respectively, countably) many accepting computations.
The degree of ambiguity of a regular language is defined in a natural way. A
language is k-ambiguous (respectively, boundedly, finitely, countably
ambiguous) if it is accepted by a k-ambiguous (respectively, boundedly,
finitely, countably ambiguous) automaton. Over finite words, every regular
language is accepted by a deterministic automaton. Over finite trees, every
regular language is accepted by an unambiguous automaton. Over -words
every regular language is accepted by an unambiguous B\"uchi automaton and by a
deterministic parity automaton. Over infinite trees, Carayol et al. showed that
there are ambiguous languages.
We show that over infinite trees there is a hierarchy of degrees of
ambiguity: For every k>1 there are k-ambiguous languages which are not k-1
ambiguous; and there are finitely (respectively countably, uncountably)
ambiguous languages which are not boundedly (respectively finitely, countably)
ambiguous.Comment: Revised according to the reviewers comment
Regular Combinators for String Transformations
We focus on (partial) functions that map input strings to a monoid such as
the set of integers with addition and the set of output strings with
concatenation. The notion of regularity for such functions has been defined
using two-way finite-state transducers, (one-way) cost register automata, and
MSO-definable graph transformations. In this paper, we give an algebraic and
machine-independent characterization of this class analogous to the definition
of regular languages by regular expressions. When the monoid is commutative, we
prove that every regular function can be constructed from constant functions
using the combinators of choice, split sum, and iterated sum, that are analogs
of union, concatenation, and Kleene-*, respectively, but enforce unique (or
unambiguous) parsing. Our main result is for the general case of
non-commutative monoids, which is of particular interest for capturing regular
string-to-string transformations for document processing. We prove that the
following additional combinators suffice for constructing all regular
functions: (1) the left-additive versions of split sum and iterated sum, which
allow transformations such as string reversal; (2) sum of functions, which
allows transformations such as copying of strings; and (3) function
composition, or alternatively, a new concept of chained sum, which allows
output values from adjacent blocks to mix.Comment: This is the full version, with omitted proofs and constructions, of
the conference paper currently in submissio
Two-variable first order logic with modular predicates over words
We consider first order formulae over the signature consisting of the symbols of the alphabet, the symbol < (interpreted as a linear order) and the set MOD of modular numerical predicates. We study the expressive power of FO 2 [<, MOD], the two-variable first order logic over this signature, interpreted over finite words. We give an algebraic characterization of the corresponding regular languages in terms of their syntactic morphisms and we also give simple unambiguous regular expressions for them. It follows that one can decide whether a given regular language is captured by FO 2 [<, MOD]. Our proofs rely on a combination of arguments from semigroup theory (stamps), model theory (Ehrenfeucht-Fraïssé games) and combinatorics
Ambiguity Hierarchy of Regular Infinite Tree Languages
An automaton is unambiguous if for every input it has at most one accepting
computation. An automaton is k-ambiguous (for k > 0) if for every input it has
at most k accepting computations. An automaton is boundedly ambiguous if it is
k-ambiguous for some . An automaton is finitely
(respectively, countably) ambiguous if for every input it has at most finitely
(respectively, countably) many accepting computations.
The degree of ambiguity of a regular language is defined in a natural way. A
language is k-ambiguous (respectively, boundedly, finitely, countably
ambiguous) if it is accepted by a k-ambiguous (respectively, boundedly,
finitely, countably ambiguous) automaton. Over finite words every regular
language is accepted by a deterministic automaton. Over finite trees every
regular language is accepted by an unambiguous automaton. Over -words
every regular language is accepted by an unambiguous B\"uchi automaton and by a
deterministic parity automaton. Over infinite trees Carayol et al. showed that
there are ambiguous languages.
We show that over infinite trees there is a hierarchy of degrees of
ambiguity: For every k > 1 there are k-ambiguous languages that are not k - 1
ambiguous; and there are finitely (respectively countably, uncountably)
ambiguous languages that are not boundedly (respectively finitely, countably)
ambiguous
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